1. Field of the Invention
The present invention relates to an ink jet print apparatus and print method with which a higher printing speed can be achieved while eliminating an irregularity of an image caused by an ejection failure at a nozzle.
2. Description of the Related Art
Recently, a variety of print apparatus employing the ink jet method are provided in fields of application such as printers, facsimile machines, and copying machines. In particular, color ink jet print apparatus for printing color images using inks in a plurality of colors are rapidly spreading because of high quality images rendered by them.
Such ink jet printing can be primarily categorized into serial type printing and line type printing. In serial type printing, an image is formed by alternately repeating printing main scans in which printing is performed by scanning a printing head having a plurality of printing elements on a printing medium and sub-scans in which the printing medium is transported in a direction crossing the direction of the printing main scans. Thus, serial type apparatus are suitable for personal use and are therefore available on the market in a great variety because they are relatively small-sized and inexpensive. In the case of a line type print apparatus, a long printing head having printing elements in a quantity to cover a printing width on a printing medium is used, and an image is completed by moving the printing medium relative to the printing elements in a direction different from the direction in which the printing elements are arranged. Therefore, line type print apparatus are frequently expensive and large-sized because their printing heads are long. However, they can perform better than serial type print apparatus in terms of printing speed.
While images in high definition and high quality are expected from ink jet print apparatus as described above another important factor is an increase in speed in order to complete printing in a shorter time. It is effective to increase an ejection driving frequency for droplets ejected from each printing element in increasing the speed of an ink jet print apparatus, and increasing the number of printing elements is also an effective approach especially for a serial printer.
When the number of printing elements is increased, it is desired that no failure occurs at any of the nozzles. When a printing head is manufactured, however, some defective nozzles are inevitably generated Defective nozzles are nozzles which significantly reduce the quality of an image, e.g., nozzles which generate a white line in a printed image. Defective nozzles include not only nozzles that are completely disabled from ejecting but also nozzles whose ink droplets ejecting direction is greatly deflected from a predetermined direction (hereinafter referred to as “deflection”) and nozzles which eject ink droplets in a quantity that is greatly different from a desired quantity (hereinafter referred to as “ejection quantity variation”). As described above, such defective nozzles are generated at a certain probability. Therefore, a problem has arisen in that the yield of printing heads is lower, the greater the number of nozzles of the printing heads manufactured.
A countermeasure referred to as “ejection failure interpolation” has already been proposed for the above-described problem. “Ejection failure interpolation” is a method in which a line to be printed by a defective nozzle is interpolated using another nozzle which prints the same line. Thus, even when there are some defective nozzles on a printing head, an image can be printed with a certain degree of normality maintained. A brief description will now be made on ejection failure interpolation according to the related art.
FIGS. 1A, 1B and 1C show a state of printing in which a desired image in the same region is formed by two groups of nozzles (101 and 102) of different types FIG. 1A shows input image data for a region printed by the nozzle group 101. Each of the grids represents a pixel having multi-valued density information, and it is assumed here that density data on the order of 25% are supplied to all of the pixels. FIG. 1B shows input image data for a region printed by the nozzle group 102, and density data on the order of 25% are supplied to all of the printed pixels.
The nozzle groups 101 and 102 may be different nozzle groups which eject an ink in the same color, and they may alternatively be nozzle groups on different printing heads which eject inks in different colors. The print apparatus may be a serial type apparatus which performs printing while moving the nozzle groups 101 and 102 to the left and right, and it may be a line type apparatus which performs printing while transporting a printing medium to the left or right.
The image data shown in FIGS. 1A and 1B are thereafter processed into a binary form and are printed by the respective nozzle group 101 and 102. As a result, as shown in FIG. 1C, a uniform image having a density of about 50% is formed on paper.
FIGS. 2A, 2B, and 2C shows a method for performing ejection failure interpolation for an image when one nozzle 101a among the nozzle group 101 shown in FIG. 1A has an ejection failure. The nozzle 101a is disabled for ejection or has a defect such as “deflection” or “ejection quantity variation” even though it is enabled for ejection. Therefore, as shown in FIG. 2A, all image data in positions associated with the defective nozzle 101a are “0” (in a non-printing state). For a nozzle 102a which performs printing in the same position as the nozzle 101a, as shown in FIG. 2B, image data having a value higher (25% higher) than a value in other regions printed by the nozzle group 102 are supplied. The image data shown in FIGS. 2A and 2B are processed into a binary form and are thereafter printed by the respective nozzle groups 101 and 102. As a result, a uniform image having a density of about 50% is formed on paper as shown in FIG. 2C.
No image defect attributable to the nozzle 101a having an ejection failure is observed on the image thus completed. It is thus possible to obtain an output image having no visible white line attributable to ejection failure which is substantially similar to the image shown in FIG. 1C printed with a printing head having no ejection failure.
Ejection failure interpolation as described above has allowed printing to be continued without any increase in output time even if there are some defective nozzles whether the apparatus is a serial type or line type.
Ejection failure interpolation according to the above-described method has problems as described below, and it has provided only insufficient results.
FIGS. 3A, 3B, and 3C show a state of printing achieved by an ejection failure interpolation process according to the above-described method performed on image data having a density of 25% printed by each of a nozzle group 101 including a defective nozzle 101a and a nozzle group 102.
It is assumed here that a nozzle 102a for performing interpolation for the defective nozzle 101a has some “deflection”. In this case, since the nozzle 102a cannot properly fill a white line even if it performs printing for interpolation as shown in FIG. 3B, the white line appears on an image as shown in FIG. 3C. Further, the ink is deposited in an amount greater than that required in a region adjacent to the white line, and the region appears as a black line as illustrated. Therefore, the image defect that appears on the output image may be more striking than that in a case wherein no interpolation is performed.
When interpolation is mutually performed between the different nozzle groups, there is a good possibility of some misalignment of a nozzle which performs interpolation from a nozzle for which the interpolation is performed, as thus described. Every nozzle has some variation in directivity. Further, a relative misalignment not only occurs at one nozzle but also occurs between the nozzle group 101 and the nozzle group 102 in not a few cases.
Further, even when there is no misalignment between the nozzle groups 101 and 102 as shown in FIGS. 2A, 2B, and 2C, in a case wherein the nozzle groups perform printing in different ink colors, a line for which interpolation for a defective nozzle has been performed can appear in a striking manner in a color tint different from that in other regions.
The ejection failure interpolation is a process which is performed to make an image defect attributable to ejection failure less noticeable. However, according to the above-described method in the related art, an interpolated portion becomes more noticeable contrary to the intention in not a few cases. That is, the ejection failure interpolation process in the related art has been insufficient to achieve effects expected from the same.